Why low-frequency ultrasound?
Ultrasound is a mechanical wave with frequencies above the consciously perceptible range of audibility for airborne sound of about 16 Hz to 16 kHz which in its propagation is linked to an elastic medium. This is where ultrasound differs significantly from therapeutic procedures based on electromagnetic waves and magnetic fields such as shortwave therapy or magnetic field therapy. The therapeutic effects of ultrasound in the tissue are described by the active thermal and mechanical component. To which degree and ratio these two components become effective, depends on the choice of acoustic parameters, especially the ultrasound frequency.
It was shown that in musculoskeletal clinical pictures the active mechanical component is accorded greater significance in the pain therapy. The particularly stimulating effects are ascribed especially to the larger particle deflection and smaller particle acceleration in low-frequency ultrasound (20 to 120 kHz, sometimes also called longwave ultrasound). Propagation and distribution of ultrasound energy in the tissue through particle interaction is also larger and more uniform.
Apart from the direct mechanical effect on the tissue by pressure and particle motion, the active mechanical component is additionally determined by triggered secondary effects such as cavitation, microstreaming and electrical phenomena. For the formation and conversion of action of the mechanical effects, there must be a sufficiently large time window for the pressure and decompression phases. The mechanical effects are often also described using the concept of "micromassage".
With subaqual application, there are still mechanical effects on the tissue surface and microorganisms that come along due to the cavitation effect in the water given an adequate ultrasonic intensity. These additional effects are advantageously used in wound therapy.
The active thermal component in tissue is much smaller as with high-frequency ultrasound (800 kHz to 4 MHz). Dangerous local overheating, e.g. on the periosteum, is safely avoided through correct application. Therefore, treatment can be done in stimulus phases depending on the case. Experience has shown that only superficial heating of the skin occurs.
Physical characteristics
Low-frequency ultrasound as opposed to high-frequency ultrasound has
• lower absorption in tissue
• greater physical penetration depth in tissue
• stronger particle deflections
• significantly better acoustic penetration and power in bone
• greater kinetic effects
• a nearly homogeneous near field and strongly divergent far field
• very small formation probability of standing waves
Therapeutic benefits of low-frequency ultrasound
As opposed to high-frequency ultrasound, various physiological effects are clearly more distinct such as the increase in microcirculation and the eutonising effect on the musculature. The effect of the increase in skin permeability persisting for several hours is also favourable. This is why topically applied hydrophilic and lipophilic medicines more easily and quickly reach the tissue and their effectiveness is thereby boosted.
1. An immediate and/or short-term analgesic effect
2. An earlier onset of action with serial application
3. A longer and persistent effect after end of treatment
4. High degree of patient safety
The most important papers for the therapeutical application of low-frequency ultrasound are specified in the list of literature. A detailed description of the basics is contained in the accompanying documents of the low-frequency ultrasound therapy devices.
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